1. Field of the Invention
The present invention relates to a lapping apparatus which is capable of improving a flatness level of a lapped surface of a flat workpiece such as a semiconductor wafer, etc.
2. Description of the Related Art
Up to now, a method of manufacturing a semiconductor silicon wafer with mirror finished surfaces is generally comprised of: a slicing step to obtain a thin disc-shaped wafer by slicing a single-crystal ingot formed by a pulling step using a crystal pulling machine; a chamfering step to chamfer peripheral edge portions of the thin disc-shaped wafer obtained through the slicing step in order to prevent it from being cracked or broken; a lapping step to flatten the surface of the chamfered wafer by lapping it; an etching step to remove processing damage in the surface of the so chamfered and lapped wafer; a polishing step to finish the etched wafer surfaces into mirror surfaces; and cleaning step to clean the polished wafer.
By the way, the lapping step is made to make the sliced wafer uniform into a predetermined thickness, as well as to obtain a required accuracy in shape such as flatness and parallelism. In general, there has been known that the wafer which has been subjected to the lapping process is the highest in the accuracy of shape, which is considered to decide a final shape of the wafer. Thus, the accuracy in shape in the lapping process is very important.
Also, as the lapping technique, there has been known, up to now, a lapping apparatus that conducts lapping in such a manner that three motions comprised of (1) rotating motion of concentric disc-shaped lapping turn tables, (2) revolving motion of circular fittings for holding workpieces with respect to the lapping apparatus, and (3) rotating motions of the circular holding fittings are combined to give relative motions to the lapping turn tables and the workpieces for lapping. The lapping apparatus is structured, for example, as shown in FIGS. 4 to 6.
FIG. 4 is an exploded perspective view explanatorily showing a lapping apparatus, FIG. 5 is a cross-sectional view explanatorily showing the lapping apparatus, and FIG. 6 is a top view explanatorily showing a state where an upper lapping turn table is removed from the lapping apparatus.
In FIGS. 4 to 6, a lapping apparatus 22 includes a lower lapping turn table 24 and an upper lapping turn table 26 which are disposed vertically opposite to each other. The lower lapping turn table 24 and the upper lapping turn table 26 are allowed to rotate in reverse directions by drive means not shown, respectively.
The lower lapping turn table 24 includes a sun gear 28 on its top surface in the center portion thereof, and an annular internal gear 30 is disposed adjacent to the peripheral edge portion thereof.
Reference numeral 32 denotes disc-shaped workpiece holding fittings (which is generally called "carriers"), and each of the workpiece holding fittings has a gear portion formed on the outer circumferential surface which is meshed with the above sun gear 28 and an internal gear 30, thereby constituting a gear structure as a whole. A plurality of receiving holes 34 are defined in the workpiece holding fittings 32. A workpiece (W) to be lapped such as a wafer is disposed in each of the receiving holes 34.
The workpiece holding fittings 32 are located between the upper and lower lapping turn tables 26 and 24, and a gear formed in the outer periphery of each the workpiece holding fittings 32 is engaged with the sun gear 28 situated in the center of the lower lapping turn table 24 and an internal gear 30 formed in the outer periphery of the lower lapping turn table 24, and the upper lapping turn table 26 is put down on the lower lapping turn table 24. The workpiece holding fittings 32 perform a planetary gear motion between the upper and lower lapping turn tables 26 and 24 that rotate while being opposed to each other. The workpiece (W) is received in each of the receiving holes 34 which are opened in the workpiece holding fittings 32 so that the rotating motion and the revolving motion are given to the workpiece holding fittings 32.
In order to conduct a lapping work, a suspension (A), which is called "slurry", consisting of polishing abrasive grains such as aluminum oxide (Al.sub.2 O.sub.3) or silicon carbide (SiC) and a liquid such as water containing a surface active agent is allowed to flow from a nozzle 36 via through holes 38 provided in the upper lapping turn table 26 into a gap defined between the upper and lower lapping turn tables 26 and 24 to feed the abrasive grains into a space between the workpiece (W) and the upper and lower lapping turn tables 26 and 24, thus transferring the shapes of the upper and lower lapping turn tables 26 and 24 to the workpiece (W).
In order to obtain a higher flatness level of the workpiece (W), it is required that the shapes of the upper and lower lapping turn tables 26 and 24 are exactly transferred to the workpiece (W) as they are, and therefore the motion of the slurry (A) between the workpiece (W) and the upper and lower lapping turn tables 26, 24 which are relatively moved cannot be ignored.
This is because the abrasive grains in the slurry (A) are constantly worn so that the diameter and the sharpness of grains are changed. If the motion of the slurry (A) is deviated, at a portion where the flow of the slurry (A) is poor, the workpiece is lapped with the abrasive grains which are smaller in diameter and lower in sharpness as compared with other portions, which causes the workpiece to be thickly lapped soon.
For that reason, grooves 40 are dug in the upper and lower lapping turn tables 26 and 24 at intervals of 20 to 50 mm in a square sectioned manner, and a useless slurry (A) or chips are exhausted from the grooves 40. However, this is not sufficient to remove the above problems under the existing circumstances.
In the above-mentioned lapping apparatus, in particular, because the circular workpiece holding fittings 32 that conduct rotation-motion and revolution-motion are exerted by a frictional force accompanied with lapping processing between the upper and lower lapping turn tables 26 and 24, and the workpiece (W), it is formed of a structural component which is very large in strength of the load.
Whereas, for example, in the newest semiconductor processing, the dimensions of the workpiece (wafer) are set such that a ratio of a thickness of the workpiece to an area thereof is very thinned with about 0.7 mm in thickness and 200 mm in diameter. The disc-shaped workpiece holding fittings 32 that hold a plurality of workpieces thus very thinned together make a huge gear which is 600 mm in diameter. On the other hand, the workpiece holding fittings 32 are strictly limited in dimension because the maximum thickness thereof is limited to the maximum thickness of the workpiece.
The thickness of the conventional workpiece holding fittings 32 is generally standardized to be thinned to the degree that it is about two times as much as the maximum grain diameter (about 10 .mu.m) of the abrasive grains in the slurry with respect to the thickness of the workpiece, for example, the wafer. In other words, it is thinner than the thickness of the workpiece by 20 to 40 .mu.m.
Although the above lapping processing is made to improve a flatness level of the workpiece (W), in order to ensure the strength of the workpiece holding fittings 32 which have a gear structure, it was a conventional general operating condition to increase the thickness of the workpiece holding fittings 32 as much as possible within the finished thickness of the workpiece (W). Under this circumstance, the workpieces (W) have been often lapped insufficiently in their surface flatness level. Also, as a wafer has become larger in diameter, a device pattern is progressively fined with the result that a higher flatness level has been required.
The present inventors studied dimensions of workpiece holding fittings that have conventionally caused a problem with an improvement in the quality of various materials and a processing technique in recent years, and investigated dimensions of workpiece holding fittings which improves a flatness level of a lapped workpiece. The present inventors paid attention to a flow of a slurry, and attempted to thin the thickness of the workpiece holding fittings 32 against a conventional taboo in order to smooth the flow of the slurry with the result that the inventors found out that the lapped surface of the workpiece can be improved with setting the thickness of the workpiece holding fittings to be thin within a predetermined range with respect to the workpiece, thus achieving the present invention.